The present invention relates to a control device for a slanting plate type variable capacity pump, and more particularly, it relates to a control device suitable for a slanting plate type variable capacity pump for supplying pressurized operating oil to an injection cylinder of a hydraulic type injection molding machine.
A hydraulic type injection molding machine is equipped with an injection device which is driven by oil pressure. The injection device includes an injection cylinder in which an injection shaft is driven by oil pressure to advance or retreat a screw in a heat cylinder, and a hydraulic pump for supplying pressurized operating oil to the injection cylinder. For such a hydraulic pump, a slanting plate type variable capacity pump is generally used. In the injection device of the injection molding machine, injection speed or injection pressure is controlled when molten resin is injected into a mold. When injection speed is controlled, resistance by molten resin in the heat cylinder and molten resin in the mold acts on the control. The injection speed decreases as the resistance increases. In particular, when the operating speed of the screw is low, there is a case that the screw is stopped. The reason is as follows.
The injection device of a hydraulic type includes a large number of oil pipes through which the operating oil passes. In those oil pipes, there are inevitable very small leakage of the operating oil. The pressure of the operating oil acting on the injection cylinder increases as the above-mentioned resistance increases. The increase of the pressure of the operating oil brings on the increase of leakage of the operating oil. As a result, the amount of the operating oil supplied to the injection cylinder decreases. In short, as injection pressure increases, injection speed, that is, the operating speed of the screw decreases and leakage of the operating oil increases. The affection of the leakage of the operating oil therefore becomes considerable when injection speed is low.
On the other hand, as the pressure of the operating oil increases, the operating oil is compressed. Although the compression quantity of the operating oil is very little relative to that of gas, the compressibility .beta. of the operating oil is represented by the following equation. EQU .beta.=-(1/V0) (dV/dP),
where V0 represents the volume of the operating oil before compression, dV represents the variation of the volume of the operating oil, and dP represents variation of the pressure of the operating oil. Since the compression quantity of the operating oil increases as the injection pressure becomes high, the affection of the compression quantity of the operating oil therefore becomes considerable when the injection speed is low.